Long-life, high reliability cryogenic refrigerators or cryocoolers such as long-life Stirling and pulse tube refrigerators are used in cooling applications including infrared sensor systems on aircraft and spacecraft. Such cryocoolers typically include a linear-resonant or reciprocating compressor having a linear electric motor to compress and displace a refrigerant working fluid. The linear motor includes a moveable, current-carrying coil, i.e., a moveable current coil, within a magnetic field to impart reciprocating, bi-directional linear forces in an axial direction to a moveable assembly of the linear motor. As the moveable assembly reciprocates in response to the linear forces applied thereto, the working fluid is compressed and displaced toward system components associated with the refrigeration process.
FIG. 1 is an illustration of a reciprocating compressor 10 which is disclosed in the publication "Design Equations And Scaling Laws For Linear Compressors With Flexure Springs", by E. Marquardt, R. Radebaugh and P. Kittel, Proceedings of the 7.sup.th International Cryocooler Conference, pp. 783, 17-19, November 1992. In FIG. 1, a left half (which is a mirror image of the right half) of reciprocating compressor 10 is depicted comprising a piston 14 driven by a linear motor, generally indicated at 16, within a pressurized motor housing 18. Piston 14 is displaced by linear motor 16 in opposing axial directions along a longitudinal axis 20 to compress a typically gaseous working fluid within a chamber 22 and to displace the working fluid through a discharge passageway 24.
In more detail, a stationary portion of linear motor 16 is fixed within motor housing 18 between a tubular left housing mount 26 and a tubular right housing mount 28. Linear motor 16 includes an outer return iron 32 fixed between mounts 26,28, and an inner return iron 30 positioned concentrically relative to outer return iron 32. A permanent magnet 34 is fixed between return irons 30,32. A moveable current coil 36, carried by a moveable armature 38 fixed to piston 14, is disposed between return irons 30,32.
An electrical outer lead 39 supplies a motor drive current from an external current source (not shown) to moveable current coil 36 through an electrical terminal 40 fixed to left housing mount 26 and a flexible inner lead 42, connected between the electrical terminal 40 and the moveable current coil 36. During motor operation, moveable current coil 36 is axially reciprocated by a magnetic force arising from magnetic interaction between moveable current coil 36, permanent magnet 34 and return irons 30,32. As moveable current coil 36 reciprocates, a moveable end 42b of inner lead 42 attached to the moveable current coil experiences corresponding movement while an opposite end 42a of inner lead 42 remains fixed to stationary terminal 40. This repetitive differential motion between fixed and moveable ends 42a,42b disadvantageously causes bending of inner lead 42 along its length which fatigues both inner lead 42 and its terminal connections to terminal 40 and moveable current coil 36. The terminal locations are especially vulnerable to fatigue failure since stresses within reciprocating compressor 10 tend to concentrate at the terminal locations.
A pair of internally mounted flexure bearing assemblies 44,46 are respectively attached to mounts 26,28 to operatively center piston 14 as well as moveable coil 36 and armature 38 within the motor housing 18. With reference to FIGS. 2A and 2B, each of the flexure bearing assemblies 44,46 includes a planar or flat flexure spring 50 that comprises an annular outer rim 52 attached to an inner periphery of an associated one of mounts 26,28, an annular inner hub 54 through which an end of piston 14 extends, and at least one resilient support arm 56 coupled between outer rim 52 and inner hub 54 through spokes 57 formed integral with the inner hub 54.
Other prior art reciprocating compressors utilize plural electrically conductive coil springs, typically four to twelve, for both supplying a motor drive current to the linear motor and aligning the moveable assembly within the compressor. In miniature cryogenic cooler applications, these coil springs must be manufactured to very close tolerances to ensure the proper operation of the linear motor. Disadvantageously, the uncompressed length of each coil spring must be several times the displacement of the moveable assembly in order to keep cyclic stresses in the coil springs to within acceptable limits. In linear motors that use a large number of springs, manufacturing cost and complexity becomes significant. In addition, although these coil springs axially center the moveable assembly, they provide negligible radial support, necessitating other means for radially centering the moveable assembly, such as a guidepin.
Another prior art reciprocating compressor 58 is depicted in FIG. 3, wherein a moveable assembly is axially reciprocated within a housing 60 by a linear motor. The moveable assembly includes a moveable mount 62 axially moveable within housing 60, a piston 64 fixed to a center of the moveable mount 62, and an armature 66 fixed to an edge of the moveable mount 62 and carrying a moveable current coil 68 concentric to the piston 64. A cylinder 70 fixed and centered within housing 60 is concentric to piston 64. A plurality of electrically conductive coil springs 71, typically four, retained between moveable mount 62 and an end 72 of housing 60, provide axial alignment of the moveable assembly within housing 60. A plurality of inner coil springs 74, typically four, assist in this axial alignment of the moveable assembly, and additionally provide a small measure of radial support therefore. A guide pin 76 constrains the moveable assembly from rotation about a longitudinal axis of the compressor. The linear motor for reciprocating the moveable assembly includes moveable current coil 68 and permanent magnet 78 fixed within the housing proximate moveable current coil 68. To operate the linear motor, drive current is supplied to moveable current coil 68 through the electrically conductive coil springs 71. Reciprocation of piston 64 compresses a working fluid within a compression space 77 to displace the working fluid through a discharge passageway 79.
One notable advantage of the flexure spring of FIG. 1 over the conductive coil springs of FIG. 3, is the ability of the flexure spring to both axially and radially align the moveable assembly of the reciprocating compressor.
Accordingly, in a reciprocating compressor, it is desirable to properly locate the moveable or reciprocating assembly within the motor housing, both axially and radially, without resorting to a plurality of conductive coil springs and a separate radial alignment mechanism.
It is also desirable to couple the electric current for driving the linear motor from an external source to an electrical terminal of a moveable coil of the linear motor within the motor housing, without resorting to a plurality of conductive coil springs or to failure prone electrical leads and lead connections.